A man weighing M kg, stands on a weighing machine inside a lift. What will be the reading of the machine if it:
(a) descends with acceleration a,
(b) ascends with an acceleration a and
(c) moves with constant velocity in upward direction?
(a)
The true weight of body = Mg
Reaction of floor = R
Net force acting on the body is,
F = Mg - R
If a is the acceleration with which lift moves downward, then
F = M a
∴ Ma = Mg - R
i.e., R = M(g - a)
Thus the apparent weight of the body is,
(b)
If lift is ascending in upward direction with acceleration a then
Ma = R - Mg
or 
(c)
If lift moves with constant velocity then acceleration, a = 0.
∴ 
So, reading on the machine = Mg
